Electrostatic photographic process of making multi-colored prints



FIG-1 Jan. 31, 1967 D L. FAUSER ETAL 3,301,675

ELECTROSTATIC PHOTOGRAPHIC PROCESS OF MAKING MULTI-COLORED PRINTSOriginal Filed June 8, 1961 2 Sheets-Sheet 1 coooooooil I IN VEN TORSDONALD L. FAUSER 8 BY EDWIN R. KOLB WW4 ma ATTORNEYS Jan. 31, 1967 I n.L. FAUSER ETAL 3,301,675

ELECTROSTATIC PHOTOGRAPHIC PROCESS OF MAKING MULTI-COLORED PRINTSOriginal Filed June 8. 1961 2 Sheets-Sheet 2 INVENTORS DONALD L. FAUSER8 BY EDWIN R. KOLB ATTORNEYS United States Patent 3,301,675ELECTROSTATIC PHGTOGRAPHIC PRQCESS OF MAKING MULTI-COLGRED PRINTS DonaldL. Fauser and Edwin R. Koib, Cleveland, Ohio, assiguors toHarris-Intertype Corporation, Cleveland, Ohio, a corporation of DelawareOriginal application June 8 1961, Ser. No. 115,643, now Patent No.3,241,957, dated Mar. 22., 1966. Divided and this application Feb. 18,1965, Ser. No. 433,816 10 Claims. (Cl. 961.2)

This application is a division of our application Ser. No. 115,643,filed June 8, 1961, which is a continuationin-part of application Ser.No. 762,756 file-d Sept. 23, 1958.

This invention relates to improvements in electrostatic photography, andmore particularly to a combination of imaging materials which arecompatible and capable of being utilized to form an image in one coloror form a m'ulti-colored image wherein each image remains substantiallyadhered to an image bearing surface.

In present electrostatic photography systems, a plate is employed havinga photoconductive coating thereon, such as selenium adhered directly toa base plate by vapor deposition, which is capable of being uniformlycharged and exposed to form an electrostatic latent image. This imagemay be then developed by use of a powder carrier technique which causesthe powder to become attracted to the charged portion of the plate, andthereafter, the developed image is transferred to the paper and fixedthereon, or one may transfer the charge image to the paper andsubsequently employ a powder developer to render the image visible,followed by a fixing operation to cause the deposited powder to adherepermanently to the paper. In a system of the above type, the imageformed on the paper by either charge transfer or transfer of the powderimage may lack the desired quality and definition, and perhaps, thedesired quality and definition can be obtained if the image weredeveloped on the surface of the photoconductor and remained thereon.

Another system which has been proposed includes the use of an imagereceiving member including a finely di vided photoconductive materialadhered to the surface of a suitable substrate by a film forming bindersuch as a silicone resin, or a urea or melamine formaldehyde resin, forexample, Suitable substrates include paper, plastic films, thin metallicfoils, and imaging members of this type are generally referred to asphotoconductive papers, although it is to be understood that the phrasepaper is not descriptive of the several substrates which are possible.

In contrast to the plate or drum systems heretofore employed, in thepaper system an electrostatic latent image is formed on theele-ctrophotosensitive surface, and the image is developed thereonwithout the necessity of transferring the powder image or the chargeimage, and, the powder image may be fixed directly on the surface of theelectrophotosensitive surface in order to impart stability to the image.The use of a photoconductive paper thus eleminates, to a substantialdegree, the need for transferring the powder or charge image to aper,however, there may still be the requirement of arranging for thedeveloped image to be fixed in order to provide a permanent image.

Recently, a developer system has been proposed which makes it possibleto develop images with improved clarity, contrast and definition. Onesuch system is disclosed in the Metcalfe article Liquid Developers forXerography, Journal of Scientific Instruments, volume 32, February 1955,pp. 74 and 75. The developer disclosed in this article represents asimple system including a dispersion of 3,3l,ii75 Patented Jan. 31, 1967finely divided carbon black in an insulating liquid. When such a liquiddeveloper, as above described, is employed under the proper conditionwith the plates, drums or papers above described, it may be necessary toemploy a separate fixing operation in order to achieve the desiredstability of the developed image.

It has been noted with hydrocarbon vehicles which are Suitable as adispersing medium for the carbon black that an adverse effect on theresins employed in the photo-' conductive pa-per frequently occurs.Brief contact between the paper'and the dispersing medium has resultedin a loss of paper stability and brought about dimensional changes dueto the swelling action of the medium on the film forming binder. It hasbeen discovered in accordance with the present invention that suchdimensional changes may be overcome by a proper correlation of thecomponents of the liquid developer and the resin employed as a filmforming binder of the paper in order to provide a compatible systemwherein the components of g the developer exhibit little, if any,adverse effect upon the image bearing member.

Accordingly, it is a primary object of the present invention to provideelectrophotographic systems or arrangements wherein an electrostaticlatent image on an image bearing member including a film forming binderis treated with a liquid developer which exhibits little or no adverseeffect on the image bearing member, and wherein marking elements of thedeveloper are substantially fixed to the image bearing member as aresult of fixing ingredients in the developer in order to provide animage which is substantially permanently adhered to the surface of theimage bearing member.

The above object has been accomplished successfully in accordance withthe present invention by employing a liquid developer in the form of adispersion of marking elements in a suitable suspension medium fordeveloping an electrostatic latent image on the surface of an imagebearing member. The developer preferably includes a liquid suspensionmedium or vehicle phase which may be called a liquid carrier, andwherein the liquid medium has an electrical resistivity of approximately10 ohm centimeters or greater. Dispersed throughout the medium is atoner including marking agents or aggregates thereof, which carry inpreferential association therewith elements of a macro-molecular binderor the medium may have dispersed therein resin components capable offorming an organosol, in which instance the marking agent may include aresin and a dye material, and charge constituents in sufficient densityto develop a zeta potential consonant with the desired electrophoreticmobility through the medium. It is preferred that the elements of thetoner include a component capable of effecting a wetting action on thebase materials, or the surface bearing the electrostatic latent image,as will be hereinafter described.

The image bearing member employed is preferably of a type includingfinely divided photoconductive material which is dispersed throughout afilm forming binder to form a matrix adhered to a suitable backingmember. The film forming resin preferably has a dipole moment which isappreciably different from the dipole moment of the suspension medium,and may be of any of the suitable water soluble resins, for example,poly acrylic and polymethacrylic acid and ester resins, .polyacrylic andpolymethacrylic amines, and polyvinyl compounds containing functionalgroups rendering them water soluble, all of which are capable ofproviding suitable electrical characteristics subsequent to depositionon a suitable substrate. Also suitable as film forming binders inaccordance with the present invention are resins which are normallywater insoluble but which are capable of being rendered temporarilysoluble by a transient reaction, as well as resins capable of beingdeposited by hot melt, organic solvent coating, or emulsion coatingtechniques.

Illustrative of other suitable film forming binders are polymers andcopolymers which may be rendered water soluble under certain conditions,are, for example, partially esterified copolymer-s, polyvinyl methylethermaleic anhydride copolymers which are partially esterified orpartially amide, methacrylic acid-vinyl acetate copolymers and otherswhich may be either soluble when neutralized to an alkaline pH but whichare not water soluble at an acid pH. The resin-photoconductive matrixdeposited on the surface of a suitable substrate, as above described,achieves a water insoluble condition to provide a surface coating whichis water insoluble at a neutral pH. Image bearing members of the abovetype are disclosed in copending applications Ser. No. 640,353, filedFeb. 15, 1957, and Ser. No. 762,699, filed Sept. 23, 1958 now Patent No.3,155,531, while developers of the above type are disclosed in copendingapplication Ser. No. 762,756, filed Sept. 23, 1958, all assigned to thesame assignee as this application.

. It is another object of this invention to provide a'system of the typepreviously described for developing electrostatic latent images with aliquid developer of the type described wherein the developer includes acomponent capable of being polymerized, and wherein the developed imageremains substantially permanently adhered to the surface of the imagebearing member due to the polymerization of a component of the developeron the surface of the image bearing member.

Another object of this invention is the provision of a system fordeveloping electrostatic latent images wherein a component of thedeveloper reacts or otherwise effects the film forming binder to causesubstantial fixing of the marking elements to the surface of the imagebearing member.

Still another object of this invention is the provision of electrostaticphotography systems employing an image bearing member wherein arelatively finely divided photoconductor dispersed in a film of binderis adhered to a suitable base, and wherein the liquid developer empolyedincludes ingredients exhibiting no adverse effect on the film formingbinder.

It is a further object of this invention to provide an electrostaticphotography system wherein an electrostatic latent image, formed on alphotoconductive matrix including a film forming binder, is treated witha liquid developer including a suspension medium such that the polarnature of the medium is substantially different from that of the filmforming binder whereby softening or swelling resulting in dimensionalchanges of the film forming binder is avoided.

A further object of this invention is the provision of a system forremoving sufficient amounts of the suspension medium employed in theliquid developer from the surface of the image bearing member in orderto form a relatively permanent image on the surface of the image bearing member, and wherein the removal of the suspension medium isaccomplished without materially interferring with the definition andclarity of the deposited image.

Still a further object of the present invention is the provision of asystem for forming a composite print by electrostatic photographywherein a plurality of latent images are formed on an image bearingmember, and wherein each image is developed by a liquid developer toproduce a composite print.

Still another object of the present invention is the provision of anovel system for forming a composite print in multi-colors byelectrostatic photography wherein a plurality of latent images areformed on an image bearing member, each representative of a differentcolor, and wherein each image is developed bya liquid developer of adifferent color to provide a composite print in multicolors.

An additional object of this invention is the provision of a system ofelectrostatic photography for producing a composite image by thesuccessive steps of forming a plurality of latent images and developingeach latent image in a desired color, and wherein a high intensity flashsource of light is employed to expose an original for a relatively shortperiod of time with a relatively high intensity light source in order toform a usable exposure.

It is still a further object of the present invention to provide amulti-colored electrostatic photography system wherein a registry systemis employed to form index marks on an image bearing member, which markscontrol the subsequent placement or location of further image componentsof the print so as to enable accurate registration of each image withrespect to the others forming a multicolored print.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

In the drawing:

FIG. 1 is a diagrammatic view of an electrostatic photography systemconstructed in accordance with this invention;

FIG. 2 is a diagrammatic view of a multi-colored system constructed inaccordance with the invention; and

FIG. 3 is an enlarged view of one of the developing systems shown inFIG. 2. i

As an aid to simplify or clarify the disclosure hereof, it may behelpful preliminarily to consider generally some factors relating to (1)image bearing members including the film forming resin andphotoconductive materials, (2) developer and toner formulations andother considerations, and (3) interaction of the developer and imagebearing members.

The term image bearing member as employed in this disclosure and as setforth in the claims is intended to encompass any member which is capableof receiving an image in charge configuration and maintain such, a configuration for a period of time sufficient to all-ow'treatment with asuitable developer. It is to be understood that such a member may be alaminated article of the type. previously described wherein one surfaceand possibly two surfaces are capable of receiving an electrostaticlatent image either by having one formed thereon directly, or by havingone transferred thereto 'by procedures well known in the art, and may ormay not include an electrophotographic surface.

The improved developers employed in accordance with the presentinvention are primarily three-component systems including a liquidsuspension medium, a marking agent and a resin and/or surfactant whichis carried in preferential association with the marking agent to form amarking complex or element. The resin and/or surfactant component may beconsidered a charge control and fixing agent operating to provide amarking complex of the proper charge with respect to the medium and theimage in charge configuration present on the surface of the imagebearing member,

IMAGE BEARING SURFACES OR MATERIALS As is well known every molecule isformed of positively charged nuclei and negatively changed electrons,and when a molecule is placed in an electrical field, the action of thefield tends to orient the molecule by attracting the positive nucleitoward the negative electrode while the electrons are attracted towardsthe positive electrode. This orientation is the result of a polarizationwhich takes place such that an electric dipole is formed, which incertain instances lasts as long as the field is applied. Other moleculesexhibit an inherent characteristic dipole-moment because the centers ofpositive and negative charge distribution do not coincide, or becausethe arrangement of the molecule is such that there isa non-equidistantdistribution of electron pairs.

In the case of organic compounds, the presence of certain functionalgroups such as alcohols, amides, amines, acid radicals, and the like, aswell as inorganic groups at one end of the molecule, will impart to themolecule a non-uniform electrical distribution resulting in a dipolemoment. In certain instances, the distribution of func tional groupsacting as electrical centers is symmetrical such that the centers arespaced an equal distance apart from the positive center. Considering amolecule wherein the two electrical centers are spaced apart a distanceI, the dipole moment, which is referred to as mu, equals Z1, wherein Zrepresents the charge magnitude. In instances where the electricaldistribution is uniform, the net dipole moment is zero.

To illustrate the above differences in dipole moment as a function ofelectrical symmetry, those organic compounds which are electricallysymmetrical are considered to have a zero dipole moment, such compoundsbeing for example, carbon tetrabromide, carbon tetrachloride,

those compounds exhibiting a high dipole moment are said to be polar,while those exhibiting a very low dipole moment, or a zero net dipolemoment, are said tobe nonapolar.

In considering the film forming resins which may be used in accordancewith the present invention, it is preferred that the resin be polar innature, or formulated to include polar monomer units, that is, having arelatively high dipole moment to provide a film coating which is stablewith respect to the components of the developer. The photoconductivematerial is generally in the form of finely divided organic or inorganicparticles dispersed in a binder to form a matrix permanently adhered toa base member. Reactions may take place between the components of theliquid developer and the film forming resin which result in dimensionalchanges of the matrix or image bearing surface. Such dimensional changesare objectionable since they may cause appreciable distortion in thefinal print, or produce other undesirable secondary effects.

Dimensional changes may result because a component of the developer isable to cross-link or penetrate within the molecular structure of thefilm forming binder, such penetration being, perhaps, a result ofhydrogen bonding by which a hydrogen atom from a component of the liquiddeveloper becomes interchanged with a hydrogen atom in the film formingbinder. If a major source of swelling of the film forming binder be thetype of compound employed as the suspension medium for the liquiddeveloper, utilization of a film forming binder which is polar in natureand a suspension medium for the developer which is non-polar in nature,operates to effect substantial reduction of such undesirable swelling.

It is to be understood that such swelling may not necessarily be aninstantaneous procedure but may progress over a period of time, andultimately an equilibrium condition is achieved wherein little, if any,additional swelling results after prolonged contact with the suspensionmedium. Thus, if equilibrium conditions are established beforedimensional changes take place in the film forming resin orbinder, thefilm forming binder may be considered compatible with the suspensionmedium or any other component in the liquid developer. It is preferredin accordance herewith that the equilibrium condition be achieved at apoint such that substantially nov swelling or.

Even if the degree of swelling is not sufficient to create anyappreciable dimensional changes in the image bearing surface, still suchswelling may bring about certain ancillary undesirable conditionsresulting in background deposits or an increase in tack which ultimatelyproduces a viscous entrapment of the marking elements of the developerin those areas of the image bearing surface which should not have anydeposit formed thereon. The tendency for dimensional changes to takeplace is of considerably significance in relation to multi-color workwherein an image bearing surface is sequentially treated with. aplurality of liquid developers as will be described more fullyhereinbelow.

As was mentioned above, every organic compound has a characteristiccharge distribution. If swelling occurs due to hydrogen bonding orcross-linking, the inherent change distribution is altered in thoseareas where the hydrogen bonding has taken place resulting in a decreasein the polar nature of the resin and possibly an alteration in theelectrical resistance of the surface of the image bearing member. If itis assumed that the surface of the film forming resin possesses aninherent positive charge, there may be an interference with thedistribution of such a charge as a result of the swelling taking place.

The inherent charge distribution, above described, is attributable tothe nature of the molecules constituting the film forming binders andgives rise to van der Waals forces or uniform weak force fields whichtend to promote adhesion of any element which is brought within theinfluence of the force fields, a feature which is desirable especiallywith respect to those marking elements attracted to the surface by thepresence of a field condition attributable to an electrostatic latentimage.

As the film forming resin swells or becomes solvated, the viscosity andmodulus is reduced as a result of the swelling, and the tack of thecoating is increased resulting in undesired viscous entrapment ofmarking elements.

From the above discussion, it can be seen that even if the action of thesuspension medium on the resin does not cause swelling to such a degreeas to result in severe distortion or separation of the film formingbinder from the substrate, there may very well take place importantancillary or secondary effects which may increase background deposit ordisturb the inherent charge relationship between the marking element andthe film so that secondary fixation attributable to van der Waals forceswill be disturbed from the desired condition.

One feature of the present invention relates to controlling thesoftening of the film forming resin or binder such that an equilibriumcondition is established before dimensional changes take place in thefilm forming binder, which softening is advantageously employed to holdthe deposited particles and accomplish fixing of the deposited developerfor providing a print which exhibits considerable stability to smudgingor smearing. In operation, the components of the developer arecoordinated or correlated with respect to the film forming binder so asto effect establishing an equilibrium condition at which the filmforming resin is softened to a 'point such that the deposited elementsof the developer are capable of being embedded or at least partiallyembedded in the surface of the image bearing member. Establishing anequilibrum condition before dimensional changes take place, and one inwhich the film forming binder is softened without causing dimensionalchanges thereof, operates satisfactorily to provide a fixed image aswill be described more fully hereinbelow and especially with referenceto the specific examples to follow.

In accordance with the present invention, film forming binders areemployed which are polar in nature to effect substantial physicalstability of the resin in the presence of non-polar suspension mediumsso that swelling, as well as the adverse secondary effects noted above,are substantially eliminated. It has been found that the resins employedas film forming binders in accordance with copending application Ser.No. 640,353, mentioned above, operate to produce satisfactory resultsand possess the desired characteristics which have been noted above.

Typical or illustrative examples of resin-s with which satisfactoryresults have been obtained in accordance herewith are disclosed in saidcopending application and it is intended that such examples beincorporated herein by reference.

It is to be understood that the above discussion applies to thecharacteristics of the film forming resin which are desirable in theabsence of any uniform potential applied to the surface of the resin asby corona charging, and the 'like, since the condition of the surface ina noncharged state will assist in controlling background deposition anddefinition during the electrophotographic process. The formation ofimage and non-image or background areas serves to attract the markingelements in a preferential manner from the developer dispersion whilethe inherent electrical condition of the film forming resin matrixoperates to maintain background deposits at a desirable low level, andthus the undesirable ancillary side effects attributable to swelling,etc., as noted above, are substantially eliminated, or minimized.

It is also possible in accordance with the present invention to employthe principles of-viscous entrapment, above described, in order to forma positive print from a negative original especially in line work,although such a system is not limited thereto. In operation, anelectrostatic image corresponding to the negative may be formed and adeveloper employed which is repelled by the image areas and deposits inthe non-image areas because of controlled viscous entrapment and theother secondary effects previously noted in order to provide a positiveprint from a negative original. In the usual process of electrostaticphotography, it is desired to have the mark ing element attracted tothose portions of the image bearing surface which are charged in orderto produce a developed image corresponding to the original. However, inthe mode of operation previously described, the charged portions oftheimage bearing surface operate to repel the marking elements and viscousentrapment is employed to effect deposition of the marking elements inthe non-charged areas. I a

As can be understood from the preceding discussion, the substrate uponwhich the electrophotographic matrix is deposited is preferably formedof a material which is relatively stable in the presence of componentsincluded in the liquid developer, that is to say, the components of theliquid developer and the composition of the substrate are coordinatedand correlated such that the substrate does not undergo dimensionalchanges resulting in destruction or dimensional changes in the imagebearing surface itself. This is especially true with respect tomulti-color work wherein the image bearing surface is successfullytreated with a series of developers as will be described more fullyhereinbelow.

In addition, it is preferred that the electrophotographic surface formedon the substrate be continuous in nature in order to prevent thedeveloper from penetrating through the electrophotographic matrix intothe body of the paper and possibly causing swelling of the fibersthereof. Such a characteristic of the coating is of interest especiallywith the squeegee system which will be described in connection with theinteraction between the liquid developer and the film forming resin.

TONERS AND DEVELOPERS As illustrative of the operation and enhancedresults achievable by liquid developer systems embodying and forpracticing this invention, the following specific exdeveloper vehicle.'to emphasize the visible characteristics of the developed amples arenoted among those with which satisfactory results in the development ofelectrophotographic images according to this invention are obtained:

Example I A dispersion of black iron oxide pigment in Varnolene (apetroleum derivative indicated as number 3039 and supplied by the SohioOil Company) was achieved by intimately admixing iron filings with aquantity of iron oxide pigment or powder (to break up agglomerations inthe latter), and then pouring the filing-oxide mixture into Varnolene,as a result of which charged iron oxide particles floated off into adispersion in which the development was conducted. Fixing wasaccomplished by overcoating the developed image, after evaporation ofthe Varnolene vehicle by forced air drying, with a water solution ofpolyacrylic acid which was then dried to a firm transparent film.

Example II A developer material consisting of colloidally dispersedgraphite pigment was prepared by dispersing the graphite in anon-conducting petroleum vehicle comprising one part by volume Oildag(-a graphite dispersion in petroleum oil supplied by Acheson ColloidsCompany) to 500 parts kerosene. For fixing the developed image,approximately 5% by weight of paraffin was incorporated in the liquiddeveloper for fixing as by fusing of the paraffin binder on the imagesurface to bind or hold the graphite development pigment particles moreor less permanently into position as deposited under the influence ofthe electrostatic field conditions.

Example 111 Finely ground Epon resin (as commercially available from theShell Oil Company) was dispersed in Varnolene, in which Epon resin isnot soluble, and an electrostatic image was developed on anelectrophotographic surface comprising zinc oxide as a photoconductordispersed in a polyacrylic acid binder matrix. The developed image wasfixed by heat fusion orthe deposited resin on the plate surface duringheat drying and evaporation of the In this example, since it was desiredimage, the intensity and contrast of the image were increased by thelithographic application of an ink. That is, 'since the surface itself,in non-image areas thereof, has the inherent characteristic of beinghydrophylic and since the Epon resin developed image has oleophylicsurface characteristics, the required visible intensity was achieved bydampening the entire surface with water and then rubbing a standardgreasy lithographic ink thereover with a cotton swab, as a result ofwhich treatment,

the ink was repelled by the non-image or undeveloped areas and wasreceived by the image areas on which was the oleophylic fused Epon resinfilm.

Example IV Example V For increased visible contrast and intensity, a dyesuch as Iosol black (commercially available from the National AnilineDivision of Allie-d Chemical and Dye Corp.) was added to the Saran resinsolution of Example IV before dispersing the solution in Varnolene. Thedye was added in a quantity to give the desired blue to blue-blackcolor,

then the dyed solution was dispersed in Varnolene and the image wasdeveloped as with Example IV, with fixing being accomplished by fusingthe resin image with heat.

Example VI As illustrative of fixing the developer material to the plateby softening of the image film surface, a liquid developer was made bydispersing one part by volume of a standard commercial Gravure red ink(identified as IPIFG-12277) in 500 parts Varnolene. An electrostaticcharge image was developed on a plate with this developer and the imagesurface air dried. The ink pigment was fixed onto the image surface bydipping the developed plate in toluene, a solvent for the bindercomponent of the image surface (in this case an acrylic acid esterpolymer known as Acryloid B-7 sold by Rohm & Haas) so that the imagesurface resin was softened sufiiciently to bind or adhere andpermanently fix theink pigment.

Example VII Using the same liquid developer as in the previous example,an electrostatic charge image was developed on a plate having as thebinder component in the electrophotograiphic surface a methyl acrylicacid vinyl acetate copolymer (commercially available Elvalan resin)instead of the acrylic acid ester of the previous example. After theimage was developed and air dried for removal of the Varnolene, thedeveloper material was observed to be fixed onto the image surface.Since Varnolene is not a solvent for the image surface resin, fixing .inthis case was obtained by the action of the dispersion solvent and/ orresin on the image surface film.

Example VIII The charge image on a plate similar to that in the previousexample (i.e., with an Elvalan resin binder in the electrophotosensitivesurface) was developed using the organosol dispersion of Saran resindissolved in mesityl oxide as in Example IV. When the developed platewas dried of the Varnolene vehicle, the image was observed to be fixedon the plate through the action of the mesityl oxide resin solvent withthe Elvalan resin in the image surface binder. In this case, the resinwas softened under controlled conditions and an equilibrium conditionwas established before dimensional changes occurred.

While the liquid developer systems above described operatesatisfactorily and produce highly desirable results from the standpointof clarity and definition, other developers have been formulated inaccordance with the present invention and constitute improvements andexpansions of the systems previously noted. It is to be understood thatthe principles herein-below discussed are equally applicable to some ofthe developers previously noted.

If a relatively simple dispersion including a carbon pigment dispersedin an oil-type binder or resin and a small amount of thinner is treatedwith an additional amount of thinner, a point is reached wherein thedispersion assumes a cloudy appearance. Dilution of the dispersion to arelatively low pigment concentration causes the resin molecule to coilup, separate from the pigment, and the pigment agglomerates orflocculates.

With some such dispersions, there is a critical value of dilutionwherein the particle or pigment becomes a nucleation condensation pointfor the resin present in the dispersion in relatively lowconcentrations, that is, at proper dilution, the pigment becomes acenter of activity with respect to any resin present in the mixture andthe resin becomes more closely associated with the pigment than with thesurrounding medium. The ability of the pig- 'ment and the like-to behaveas a nucleation condensation-point is a function of the mediumsurrounding it as well as other materials present in the medium, forexam- .ple, charge control and fixi-ng'agents. Once the resin beisformed composed of an element carrying with it in preferentialassociation components which may then function as a charge control andfixing agent. It is to be emphasized that the association may resultfrom the formation of a well defined envelope, while on the other handsuch an association may be the result of a density distribution of theresin and the like by which high density areas are located in closeproximity to the element or particle.

An improved developer composition in accordance with the presentinvention is a system wherein a marking agent, preferably a pigment,carries a charge control and fixing agent in close associationtherewith, as has been noted above, and wherein the above components arecapable of dilution to a degree suitable for deposition on a sur face ascontrolled by an electrostatic latent image. During such dilution fromthe original paste-like consistency, there may result a physicalrelocation of the charge control and fixing agent, and the pigmentpresent in the dispersion is able to function as a nucleationcondensation point, as above described.

In accordance with the present invention, a developer includes asuspension medium which surrounds a marking complex which is preferablyformed of a marking agent, usually a pigment particle, acting as anucleation condensation point whereby the charge control and fixingagents are more closely associated with the marking agent than with thesurrounding medium. The suspension medium is static with respect to theelectrical field present on the substrate while the complex or markingelement has electrophoretic properties which provide mobility throughthe medium. The complex is of a proper small size so as to eliminate anyappreciable viscous drag which may interfere with the electrophoreticmobility of the complex. It is preferred that the medium have a resistivity of approximately 10 ohm centimeter, or greater. Thus, the mediumpossesses an electrical resistivity which is suificiently high so as tomaintain the electrostatic latent image in well defined patterns forinteraction with respect to the complex or marking element which is tobe deposited thereon.

Materials which have been found suitable as a suspension medium includeliquids having low vapor pressure, low flash points and low toxicity.The liquid should be chemically stable to the atmosphere while beingelectrically and thermally stable at ambient atmospheric con ditions.Moreover, the liquid should be non-solvent and non-corrosive especiallywith reference to the film forming binder so as to eliminate swelling,as previously noted, as well as stress cracking or crazi-ng.

The suspension medium preferably includes elements capable offunctioning as (1) a diluent or mixture of diluents, or a diluent and asolid which is soluble in the diluent and which is inert with respect tothe electrophoretic mobility of the marking element complex, and (2)surfactants .present in optimum concentrations so as to control lateralmovement of the marking complex as will be discussed hereinbelow.

The complex or element possesses an electrophoretic mobility consistentwith the transport time in the developing operation, which is ultimatelya function of the processing time. Stated another way, the complex orelement should be selected so that it will migrate towards the imagebearing surface within the time allowed for development as determined bythe time requirements for overall processing. It is preferred that themarking agent include a particle or pigment or other material insolublein the surrounding envelope, or aggregates thereof, which carry a resincomponent in close association therewith and charge constituents insuflicient density to form a marking element having a zeta potentialconsistent with the desired electrophoretic mobility as determined bythe transport time associated with the developing procedure, andconditions of the charge image, etc.

The marking agent possesses an inherent electrical charge with referenceto its surrounding medium which may be opposite to the charge on theimage areas of the image bearing surface. The developer compositionitself, however, includes components capable of coacting with the resinsurface of the image bearing member for fixing the marking agentsdeposited thereon. The envelope surrounding the marking agentspreferably includes components such as surfactants and resins which mayserve to modify the charge on the marking agents and coact with theresin surface of the image bearing member upon deposition of the markingagents and envelope to fix the agent to the surface once a substantialamount of the suspension medium has been removed.

A satisfactory but less preferable marking agent composition includesthe use of a number of different type particles or pigments in a mediumso balanced with respect to the mass charge and viscous drag through themedium that the deposition thereof is effected simultaneously within thereference time of the process.

Considering the relationship between a particular medium and aparticular marking element, the two are coondinated or correlated sothat the fluid viscosity of the medium is such that little, if any, dragis created which opposes the electrical forces giving rise to transientadhesion or secondary fixation as has been heretofore described.

The charge modifying or control agent, as the term suggests, is employedprimarily to 'vary the charges on the marking agent to allow formulationof liquid developers wherein the composite marking element is of thedesired charge with reference to the charge on the image areas on theelectrostatic latent image. For example, if an electrostatic latentimage is formed wherein the image portions are negatively charged, apositively charged marking complex is normally employed for development,and accordingly, the charge modifying agent cooperates with a particularmarking agent to provide a positively charged complex which is depositedout of the surrounding medium in accordance with the negative chargesmaking up the electrostatic latent image. In the case of anelectrostatic latent image formed of positive charges, the control agentacts to provide a negatively charged complex which is depositedselectively out of the surrounding medium onto the positively chargedlatent image.

If desired, a charge control agent may be employed which performs twofunctions, the first being change control and the second involving theability of the charge control agent to cause fixing of the markingelement on the surface of the image bearing member. While one componentmay be employed to perform the two functions previously noted, it is tobe understood that the two separate components may be employed each ofwhich performs one of the above functions, or two components may beemployed which coact to perform the functions above noted.

In discussing the mechanics of the developer formulations, it may behelpful to consider certain principles of surface chemistry, especiallythose relating to the operation of driers and surface active agents orsurfactants. In the simple system of carbon black in carbontetrachloride, it has been observed that considerable flocculation oragglomeration of the particles occur, and a true stable dispersion ofthe carbon black is not obtained. If, however, a surface active agent isintroduced into the dispersion along with the carbon black, it isobserved that a stable dispersion is attained. Such a result isunderstandable since the surface active agent acts to wet the surface ofthe particle by reducing the surface tension or interfacial tensionso asto provide a stable suspension.

Another phase of the mechanics involved in the use of a surfactantrelates to the chemical structure thereof. Normally these surfactantsare relatively long chain organic acids for example, oleic, tiglic,lineolenic, linoleic, or n-aphthenic acids, or the metallic saltsthereof such as copper, manganese, lead, zinc, cobalt, nickel, andchromium. In addition to the above, the following operate satisfactorilyas surfactants and include octoates, tall oil, phosphoric acid,lecithin, malic acid and the metallic salts thereof, mono-, di-, andtri-ethanolamine, ethylamine and di-, tri-homologs, quaternary organiccomplexes and other compounds well known in the art.

As the surfactant is introduced into a system containing a dispersingmedium and a marking agent capable of functioning as a nucleationcondensation point, the surfactant may be absorbed by the marking agent,or it may orient itself such that the cation or anion portion of thesurfactant is more closely associated with the agent while the anion orcation portion, respectively, is orientated into the dispersing medium.

As a result of the orientation of the surfactant or the absorption ofthe surfactant by the agent, the composite of agent and surfactantacquires a characteristic charge which is the same for each composite inthe dispersion, and the composite may be conveniently referred to as amarking element. Because each marking element has the samecharacteristic change, there is an electrical repulsion between theelements of the dispersion which asslsts 1n maintaining it in a stable,flocculation-free state which resembles in physical appearance a stablecolloidal dispersion.

I The above described three component system includ- 1n g a dispersionmedium, a marking agent and surfactant, which latter in effect acts as acharge modifying or control agent, may include components which servemultiple functions as was previously stated. The surfactant in additionto providing a stable dispersion of marking elements throughout theliquid medium may also functron as a fixing agent for providingadherence of the mark-mg element to the surface of the image bearinmember. a

It is preferred in accordance with the present invention that thesurfactant be compatible with the image It is well known that somesurface active agents in addition to providing a wetting action andcontrolling the charge also act as driers promoting polymerization ofany res n component having a reactive position. By way of illustration,and in no way to be construed as a llmitation on the present disclosure,a marking element ncluding an alkyd resin and a marking agent dispersedin a liquid medium may possess the proper charge characteristics as wellas provide a stable colloidal dispersion. Use of such a liquid-developeron an image bearing surface may result in a fixing time which is toolong for the particular required processing time. In this instance, itmay be desired to incorporate a drying agent as a component of themarking element in order to accelerate polymerization of the resin. Insuch a case, the drying agent cooperates with the other components ofthe marking element to supplement the charge characteristics of theresin, and thus, the charge on the composite element. Tests of a systemas above described have indicated that as long as the image bearingsurface remains immersed in the developer solution, relatively littlepermanent fixing of the marking element on the surface of the imagebearing member occurs, however, as the surface is removed from thedeveloper and excess suspension medium is removed, the marking elementexhibits considerable stability with resistance to smudge or smearing.

As was mentioned previously, the surfactant orients itself with respectto the marking agent in a predetermined fashion such that either thecation or anion portion thereof is extended into the surrounding medium.In the case of a highly polar surrounding medium such as trichloemploy asuspension'rnedium which is anon-polar liquid such that the cation oranion portion of the surfactant which projects'into the suspensionmedium is maintained in that condition and capable of coacting withportions of the film forming binder on the surface of the image bearingmember. f i v I Typical examples of a developer formulated in accordancewith the present invention would include. liquids such as Varnolene,mineral spirits or the like, including primarily aliphatic hydrocarbonshaving a flash point between approximately 100 and 130, The solventstrength of the diluent is preferably between 23 to 40'as determined bythe kauri-butanol values and possess an aniline point of about 130 toI90. Thedilue'nt may contain about 0.5% .water and has a specificresistivity'in the order of or 109-ohm'centimeters. Aromatichydrocarbons generally exhibit'an appreciable solvating power due to thepresence of unsaturated bonds, or their more polar nature,

as is apparent from the fact that they possess a kauributanol value ofthe order of 72 to 110. Accordingly, in

employing aromatic hydrocarbons there is an increased tendency forhydrogen bonding to occur resulting in swelling as was previously noted.

Surfaceactive agents which may be employed in addition to thosepreviously'mentioned include Zinc naphthenate, zinc' linalls, manganeseoleates, linolenic and abietic acids and salts and mixed salts thereof;A convenient' source of pigments of desired small size is a BradenSutphin ink K'-325 (black). To the solvent or diluent there is addedapproximately 0.5% by weight of 'a solution of a manganese saltcontaining 6% metal ion, previously noted, in a suit able'diluent. Tothis mixture is added 0.4% to 1% by weight of the Braden Sutphin black.The above example illustrates the type ingredients employed for a usabledeveloper formulated in accordance with the present invention.

The Braden Sutphin black above described could be considered a tonerconcentrate for the developer and the following formulations provide agroup of resins (other than the Braden Sutphin black above) forsatisfactorily formulating a pigment paste or toner concentrate for use'in providing liquid developers in accordance herewith. Merely asillustrative:

Example IX ExampleX Another such resin is an alkyd resin made up of 6 0to Example X1 A third resin includes an alkyd resin of about to 65%mixed linseed oil fatty acids, the remainder being glycerol and phthalicanhydride in molecular ratios of 1.05 to 1.15 molecules of glycerolper-molecule of phthalic v anhydride. 4 v

Example XII Another resin'includesi' an alkydprepared so asto be ahydrocarbon 'soluble'resin of acid value less than 15 .and viscosityless than E at? 40% .nOn-volatile content when the viscosity is rated inGardner-Holdt scale at 77 F.

Example XIII Still another resin includes an oleo resin varnish vehiclecomprised of rosin modified phenolic resin and mixed linseed and tungoil in proportions such as to give 70 to gallons of mixed oil per poundsof varnish. The resin is heat blended and bodied at elevatedtemperatures in excess of 525 F. until the resin oil varnish blend has aviscosity greater than A but less than C at 40% nonvolatile content inmineral spirits having a kauri-butanol value equal to 35, the viscosityhaving been measured on a Gardner-Holdt scale at 77 F.

For the black toners, the marking agent is preferably selected from lampblack, furnace black or the like, having a surface area characterized bythe BET method of greater than 40 and less than 80 square meters pergram. Such a pigment has a particle size less than .1 micron average. Inthis respect, it should be noted that such a small particle, in theproper environment, provides a 'highly"reactive center or nucleationcondensation point for attracting the resin or surfactant present in thedeveloper.

A toner concentrate was satisfactorily formulated by taking one of theabove resin components and treating such a resin as follows:

Step 1.The resin is treated at elevated temperatures to a molten stateand diluted with 30 to 50 parts of its own weight with varnish, andmineral spirits having a kauri-butanol value of about 35 to 40.

Step 2.About 2 ounces of pigment were charged into a liter capacity ballmill adding sufficient vehicle (solution of resin from Step 1) toproduce a cascading action in the ball mill. Subsequent additions ofvehicle (solution of resin) were made as needed particularly to preservethe cascading action since the material thickened during the ball milloperation. When the cascade was stable, the grinding continued until theshading strength became constant. The shading strength was measured byblending one part grinding paste with one part zinc oxide in linseedoilprepared in proportions of 2 parts of pigment grade zinc oxide byweight to one part linseed oil.

Step 3.--When an equilibrium was reached, the grinding paste was dilutedwith the vehicle (solution of resin as per Step 1) until there wasestablished a pigment binder ratio of about 2.8 by weight of carbonblack per 100 parts of-resin solids.

Step 4.-To the product of Step 3, was added a solution of drier metalsparticularly 6% cobalt phthalate, 6% manganese phthalate, and 24% leadphthalate in sufficient proportions to adjust the ratio of metal contentto resin solids by weight to approximately 0.5% lead, 0.05% manganese,and 0.05% cobalt.

Step 5.-A blend of 6% manganese linoleate solution in a suitable solventwas prepared so as to provide a mixture of fluid and salts containing99.5% solvent and 0.5% of the manganese salt. 7

Step 6.To 99.0 to 99.6 parts by weight of the blend prepared by Step 5was added approximately 0.4 to 1 part of toner concentrate. The mixtureresulting from this series of steps included a dispersion of finelydivided marking elements in a suitable suspension medium. Such adispersion as prepared above operates quite satisfactorily as aliquid-developer in electrostatic photography with the advantages aspreviously noted.

As illustrative of other satisfactory pigments for use in accordanceherewith, the following may be noted: organic bentonites, bone black,charcoal black, mineral and iron oxide black varied in percentages so asto provide a surface area corresponding to the surface area of thepigments referred to above. As illustrative of color toners for useherewith. pigments such as azo yellow having a pigment concentration ofabout 15%; cyanine (blue) phosphomolybdenic toner, about 12% pigmentconcentration, magenta (red) phosphomolybdenic toner 12 to 15% pigmentconcentration, with a ratio of pigment to resin solvents as noted abovein connection with the black toners.

Other pigments which have been found suitable in accordance with thepresent invention include antimony Vermilion, barium yellow, blueverditer, bone black, cadium red and yellow, cerulean blue, charcoalblack, chrome green and yellow, cobalt green, blue violet and yellow,Egyptian blue, emerald green, black and red iron oxide, manganese blueand violet, titanium dioxide, zinc oxide and carbon black. It ispreferred that the above pigments be of a size in the order of 0.01 tomicrons and preferably in the range of 0.01 to 5 microns in order toprovide fine grain image.

INTERACTION OF THE LIQUID DEVELOPER WITH THE FILM FORMING RESIN If anelectrostatic latent image is formed on an image bearing surface in anyof the conventional methods heretofore employed, differentially chargedimage and non image areas are present on the surface of the imagebearing member having a configuration substantially identical to that ofthe developed image. During development the marking'elements are desiredto be deposited on the image areas, and only the image areas, and remainthereon as a permanent or at least substantially permanent depositformed on the surface of the image bearing member in imageconfiguration. To this end, the developer includes a suspension mediumhaving dispersed therein a finely divided agent, and preferably apigment which carries in preferential association therewith a resin and/or surfactant.

The marking agent is present either in the form of a pigment or otherparticle surrounded by an envelope of resin and/ or surfactant, orclusters of individual particles surrounded by an envelope of resinand/or surfactant. The complex element, thatis, the marking elementabove described carries an appropriate charge which has a polarity suchthat it is capable of being attracted to those portions of the'imagebearing surface which have been defined as the image areas.

As the complex or the marking element, as above described, comes intocontact with the image bearing portion of the image bearing surface, oneof two things may occur. If, in the presence of the thinner there is notendency of the envelope to wet the coating, the complex will bedeposited on the paper. as a unit and generally retain the shape it hadin the developer dispersion,

and may be easily dislodged resulting in smearing and loss ofdefinition. On the. other hand, if the wetting ability of the complexfor the surface is strong'enough, the envelope surrounding the particles'or clusters partially flattens out or spreads on the surface. If it beconsidered that the internal angle between the surface of the paper anda linetangent to the surface of the complex at the point of contact withthe. base is theta, then with an increase in time the .theta angledecreases as the resin continues to deform due to the wetting ability ofthe complex with respect to the binder. In a relatively :short time, thecomplex flattens out until a condition is reached wherein the-van derWaals forces attract the components of the complex to the surface of thefilm forming binder. At this point, the resin envelope covers themarking agent causing it to adhere to the film forming binder in asemi-permanent transient condition. Additionally, the transientcondition is characterized by an attraction between the charged complexand the image area, and tests have indicated that while in contact with.

the developer medium, some marking elements in accordance herewith maybe removed by brushing them off although there remains on the surface aconsiderable quantity of marking elements retained thereon in imageconfiguration.

As a substantial amount of the suspension medium evaporates or isotherwise removed, the resin and/or surfactant of the complex coale'scesand is polymerized to cause substantially permanent cohesion andadhesion of the marking element to the surface of the film formingbinder. Since the theta angle has decreased, due to the wetting abilityOf the resin envelope for the resin film forming binder, the electricalcharge condition of the binder molecules acts on the pigment oraggregates thereof to attract the same to the surface of the filmforming binder. Accordingly, the pigment is "virtually covered by alayer of resin which causes it to remain permanentlyadhered to the imagebearing surface.

The wettability of the complex towards the image bearing surface shouldbe at an optimum value so as to provide a controlled lateral mobilityalong the surface of the film forming binder. If the complex is notcapable of wetting the surface upon which it is deposited, the desirablesecondary effects and fixing previously noted will not be achieved, andthe marking elementmay be dislodged quite easily. On the other hand, thewettability and flow of the complex should not be so great as to causeexcessive lateral spreading since this interferes materially with theresolution of the finalprint. Employing developers and image bearingmembers coordinated in accordance with the present invention, resolutionin excess of lines per millimeter has been achieved.

As the image bearing member is withdrawn from contact with the liquiddeveloper, there remains on the image bearing surface a relatively thinfilm of the suspension medium as well as the marking elements which havebeen deposited in image configuration, as above described. The very thinfilmof the suspension medium should be substantially removed to providea virtually dry print. In accordance with the present invention it ispossible to use a squeegee roller in providing removal of considerableamounts ofthe suspension medium without effecting removal of significantamounts of the marking elements from the image portions of the imagehearing surface. s a

As was stated previously, providing a suspension medium; which isnon-polar and a film forming binder which is polar represents an optimumcondition. As a corollary to this, the squeegee roller is formed with anonpolar surface material exhibiting electrical characteristicscomparable to that of the suspension medium. This represents an optimumcondition since the dispersion and image deposit are stable with respectto the roller. Additionally, if the resin envelope of the markingcomplex is stable, it splits away from the roller at a low shear levelcharacteristic of the suspension medium to eliminate any --vis-eous dragbetween the roller and-the deposited resin envelope. By employingaroller material which is non- -polar, the formation of ghost images onthe squeegee roller is substantially eliminated, and thus, a squeegeeroller may be employed to remove significant amounts of the suspensionmedium without materially interfering with the marking elementsdeposited out of the developer.

Another advantage attributable to a squeegee operation may be understoodwith reference to multi-down or polychroniatic work wherein.multi-colored images are to be formed, as will be describedhereinafter. The application of pressure by the squeegee roller operatesto compress or coalesce the deposited resin envelope to a substantiallyuniform thickness so that penetration of light necessary for theformation of charge images representative of the different colors willbe uniform in those areas covered by a resin envelope. Thus, it canbeseen that the use of a squeegee which does not remove significantamounts of the deposited marking elements and which operates to compressthe resin envelope to, a uniform thickness may be employedadvantageously in polychromatic'or multi-color work.

MULTI-COLOR PRINTS BY ELECTROSTATIC PHOTOGRAPHY pounded as previouslydescribed is extremely advantageous when considered in reference topolychromatic work wherein an electrophotographic process is employedfor producing multi-colored prints. As has been stated, the markingelement above described when deposited on the image bearing surfaceremains thereon in substantially permanent fashion due to polymerizationof the resin or combination of resin and surfactant, or due to thereaction of a component of the developer with a component of the filmforming binder.

The unusual stability achieved by the combinations above describedreadily lends itself to a system wherein a series of electrostaticlatent images are formed each representative of a different color, andwherein each image is developed by a liquid developer of appropriatecolor to produce a mu'lti-color print. As will be hereinafter described,the steps involved in forming a multicolor print include forming a firstimage, developing that image, forming a second image in registry withthe first and having a charge configuration representative of a secondcolor and developing that color, and so forth, until a multi-color imageis formed. During the development of each of the images, the markingelements deposited in the previous development step are exposed to thesuspension medium but are not adversely affected thereby since themarking agent is permanently adhered to the surface, as above described.

By the polychromatic system of this invention, a transient fixation ofthe marking element is achieved, as previously noted, and therefore itis possible to form and develop a series of images without theadditional step of separately fixing each image after developmentthereof. Additionally, due to the compatibility of imaging materials,the image bearing surface is unaffected by repeated contact with thesuspension medium previously described.

A singular and noteworthy advantage of the color technique of thisinvention lies in the fact that the marking elements carry or areaccompanied by components capable of causing self-fixing of the markingagents to the surface of the image bearing member, and accordingly,those portions of the surface wherein there is no deposit of markingelements retain the electrical, physical as well as chemical propertiesthey had originally. Moreover, by control of the material constitutingthe film forming binder as well as the components of the developers,background deposits are substantially eliminated thus maintainingbackground free of deposits which may interfere with subsequentdeveloping operations.

All of the above advantages are of considerable importance in forming aprint in a single color, and as will be apparent, the several advantagesabove enumerated are of even greater importance in multi-color workwherein the image bearing surface may be treated with as many as six orseven sequential developing operations to provide a print of six orseven colors.

It should be noted that the liquid developer and the film formingbinder, if coordinated properly as above described, provide a compatiblesystem wherein the image bearing surface retains remarkable dimensionalstability in addition to substantial reduction in undesirable secondaryeffects, previously noted, thereby allowing repetitive contact betweenthe image bearing surface and a suitable liquid developer forrnulti-color work without materially interfering with thecharacteristics of the image bearing surface or the definition,contrast, etc., of the deposited image or images.

SINGLE COLOR SYSTEM AND APPARATUS A system incorporating the aboveadvantageous relationships previously described may be understood withreference to FIG. 1 which illustrates in schematic form apparatusembodying and for practicing the present invention. A roll ofphotoconductive paper or the like in dicated at 12 as representing acontinuous web of paper or other suitable base material is provided witha suitable electrophotosensitive surface layer of the type described inthe copending application Ser. No. 640,353, previously mentioned. Fromthe roll 12, the web 14 is withdrawn for feeding through the machine, asfor example, by passing over a guide roll 15. A registration punch 16 orsimilar device for controlling or metering the feed of web 14 inincrements commensurate with the linear dimension of whatever image isdesired to be produced on the surface of the web 14 cooperates with aweb length measuring roll 18 for controlling feeding of the web 14,either incrementally or continuously through the apparatus. Aregistration mark reader and imaging control 19 acts as a furthercomponent of the web feed control elements of the apparatus.

In the apparatus illustrated in FIG. 1, the electrophotosensitive orphotoconductive surface is on the inside surface of web 14 (i.e., thelower surface of the horizontal portions of web 14) and thiselectrophotosensitive surface is given a uniform charge by a corona unit20 through which the web passes. Positioned in close contact with thereverse side of the web 14 directly opposite the charging unit 21 isgrounded a metallic plate 22 which is of some assistance in providing auniform charge on the surface of web 14. After leaving the coronacharging unit 20, the electrophotosensitive web with a uniformelectrostatic charge pattern thereon passes into the exposure area ofthe machine indicated at 23 where a light image 24 is projected onto theweb as by a projector 25 having lens elements 26, and condensers 27 fora source of light 28 for project-ing the light image 23, for example, aphotographic transparency indicated at 29. The light source 28 ispreferably of the flash exposure type so as to provide a high level ofillumination in a relatively short time thereby facilitating operationon a continuous basis,

since the transparency may be flashed in a short interval of timerendering it unnecessary to maintain the web stationary during theflashing operation. The projection of the light image 23 onto thecharged surface of Web 14, of course, alters the uniform electrostaticcharge therein which was produced by the corona charging unit 20, andproduces on the Web 14 a latent electrostatic charge or patternincluding differentially charged image and nonimage areas.

Immediately after exposure to this light image, the web 14 is advancedto a developing station generally indicated at 30, and the web passesover an insulating guide roll 32 and around a driving capstan 34 whichimmerses the image and non-image areas of the web 14 in a liquiddeveloper composition 35 contained in a developer tank 37, whereindevelopment of the electrostatic latent image on web 14 occurs inaccordance with the principles of this invention. Still driven by adriving capstan 34, the web 14 continues over rolls 39 and 40, tocontact a heated drying drum 42 around which web 14 travels for thedrying removal of the suspension medium allowing the marking elements toremain substantially permanently adhered on the surface of the web.Thereafter, the web 14 leaves the drier, passes over guide roll 44 andonto a sheeter or other apparatus for subsequent operations. A hood 45is preferably provided around the drying drum 42 with a suction vent 46to aid in the evaporation and removal of volatile components as the webpasses taround drying drum 42.

If desired, the developing station may include a counter-electrode, ordevelopment electrode of the type described in application Ser. No.762,756 previously mentioned or a liquid counter-electrode of the typedescribed in copending application Ser. No. 96,436, filed Mar. 17, 1961now abondoned.

It is preferred that a squeegee roller be employed to remove significantamounts of excess suspension medium which adheres to the surface of theweb 14 which has been treated with a liquid developer. The details andoperation of the squeegee roller will be discussed more fullyhereinbelow in connection with the multi-color system.

19 MULTI-COLOR SYSTEM AND APPARATUS Production of multi-color prints inaccordance with the present invention may be understood with referenceto FIG. 2 which illustrates apparatus embodying and for practicing thepresent invention. A roll of photoconductive paper, or the like,indicated at 50 as representing a continuous web of paper or othersuitable base material is provided with a suitable electrophotosensitivelayer of the type described in copending application Ser. No. 640,- 353,previously mentioned. From the roll 50, a web 52 is withdrawn forfeeding through the automatic web tensioning device generally indicatedat 55 including guide rolls 57 and floating roll 58 for maintaining theunwind tension constant, and infeed rolls 59 which act to meter the webenterting the machine so as to maintain a constant and uniform webtension throughout the travel therethrough. The web tensioning controlunit 55 is of conventional design and operates to maintain the webcentered during treatment as well as maintaining the proper tension onthe Web as it passes from station to station. The web control system mayalso be used for controlling feeding of the Web 52 either incrementallyor continuously through the apparatus as will be described more fullyhereinbelow.

The first operation involved in forming a multi-color print takes placeat the first station wherein the surface of the image bearing member ischarged to a uniform potential, as is well known in the art. This isaccomplished by passing the web over roller 61 and through a corona unitindicated at 65 which is of a type well known in the art. A conventionaladjustable power supply may be provided for establishing a relativelyfine balance between the corona currents for providing an optimum chargecondition on the surface of the paper. The electrophotosensitive,photoconductive surface or image hearing surface is on the insidesurface of the web 52 (i.e., the lower surface of the horizontalportions of web 52), and as the paper is advanced from the firstcharging station 65 to the first exposing station generally indicated at67, a light image 68 is projected onto the image bearing surface by asuitable projection unit 70.

The projection unit may, if desired, include a variable magnificationfeature, and includes a flash tube capable of, for example, four 160watt second flashes per second. During this exposure operation, anelectrostatic latent image is formed having an image configuration ofthe first of a series of colors which will ultimately comprise amulti-color print. In addition to an electrostatic latent imagerepresentative of a particular color, there is also formed anelectrostatic latent image representative of an index mark which servesas a reference mark for actuating the projection units in the subsequentstations, as will be more fully described below. The index mark ispreferably formed along or near the margin of the web in a predeterminedrelation to the color image thereby providing a convenient referencemark which may be used to orient the several images with respect to eachother and in proper relationship in accordance with the finalmulticolored print.

Subsequent to the exposure of the first of a series of electrostaticlatent images, the web 52 carrying the first electrostatic latent imageis advanced to the first developer station 75 of a series of developerstations. The image bearing member carrying the electrostatic latentimage in color configuration and an image of the index mark is advancedover an insulating roller 76 and between a pair of rollers-78 and 79 atwhich point the image bearing surface is treated with liquid developercontained in the tank 80, or other suitable container. Positioned inrelatively close relationship to roller 78 is a squeegee roller 82 whichoperates to remove substantially all of the suspension medium adhered tothe surface of the web 52.

The web carrying the developed image is then passed over rollers 83 and84 to a second station including char-ging unit 90 wherein the web isgiven a second uniform charge by a corona unit similar to thatpreviously described. As the charged web is passed around insulatingroller 92, it enters the second exposure zone generally indicated at 95,and as the index rnark moves past the photo-electric register control96, the second projection 98 is energized to expose the charged imagebearing member to a second image having a configuration representativeof a second color, and properly registered with respect to the firstimage. The photoelectric register control is capable of reading towithin 10.0005 inch to produce a pulse which is fed to an amplifierproviding a suitable output for control of the flash tube containedwithin each projection.

In similar fashion to that above described, the Web is advanced to asecond developing station100 which contains the same elements as doesstation 75 with the exception of the liquid developer which is of thedesired color in accordance with the configuration of the electrostaticlatent image formed in the second exposure zone.

In this manner, the web is advanced in succession through a thirdstation including a third charging unit 102, and thence to an exposurezone wherein the index mark is recognized by the second register control107 to trigger the third projection unit 109. In the third developingstation 110 containing the same elements as described in connection withdeveloping stations 75 and 100, the third electrostatic latent image isdeveloped in a third color, and thereafter the web is advanced to afourth station generally indicated at including a charging unit 117, anindex register unit 118 which controls operation of the fourthprojection unit 120. The web carrying the fourth electrostatic latentimage is advanced to the fourth developing station 122 wherein thefourth electrostatic latent image is developed as described inconnection with the three preceding images.

The last station 125 includes a charging unit 126, an index registerunit 127 for triggering the projection unit 129, and a developingstation 130 for developing the fifth electrostatic latent image in amanner described in con- 'nection with the first electrostatic latentimage. As the web having the multi-colored image deposited thereonleaves developing station 130, it is advanced over a cutoff compensatorroller and advanced between pull rolls 136 to a sheeting assembly 140including a cut-off control 141 at which point the web is severed toprovide individual multi-coliored prints. The sheets are advanced into astacking assembly generally designated 145 which delivers the individualsheets to a delivery unit 148. If desired, an exhaust hood may beprovided for removing any odor which may bepresent due to the nature ofthe liquid developer or any constituent thereof.

It is desired at each exposing station that the web follow a slightlycurved image path or be pulled across and in contact with a surface forsubstantially eliminating transverse vibration and curling.Additionally, the apparatus may be operated in a continuous orincremental manner to provide duplicate copies of the samernulti-colored print or copies of different multi-colored prints,depending on the need.

Each of the projection units 70, 98, 109, 120 and 129 is provided with atransparency or microfilm having an image thereon representative of theimage in each color so that during the first exposure, an electrostaticlatent image is formed corresponding to the red portions of the finalprint, for example. In similar fashion, the second, third, fourth andfifth units may contain transparencies capable of providing imagescorresponding in configuration to the yellow, blue, green and blackportions of the final print so as to provide a multi-colored imageincluding the colors red, yellow, blue, green and black, for example. Itis to be noted that the image on the transparencies or microfilm needonly be in black and white since the color of the particular image isdetermined by the color of the developer. It is to be understood thatthe sequence of depositing the colors as well as the combination ofcolors may be varied in accordance with the particular need, and theabove colors are cited by way of illustration only and are not to 'beconstrued as a limitation upon the present invention.

It is also possible in accordance with the present invention tooverprint or deposit one color on another to form a third color. In thismode of operation an electrostatic latent image is formed having aconfiguration representative of a first color, and is developed in afirst color. Thereafter a second electrostatic latent image is formedwhich overlaps at least a portion of the first developed image, and thesecond image is developed in a second color. The overlapping portion ofthe two images is of a color different than the first and second color.For example, yellow may be overprinted with magenta to give a red in theoverprinted areas, or yellow may be overprinted with Cyan to give greenin the overprinted areas, etc. As the second developer is deposited incontact with the first, the marking elements of the second developercoact with the deposited elements of the first developer and operate tomaintain the second deposited elements substantially permanently adheredto the first and to the surface of the image bearing member.

In a continuous operation, the frequency of exposures at the firststation is programmed or otherwise arranged to form a given number ofelectrostatic latent images per unit length of the web. The formation ofeach of the subsequent images is controlled by the index or referencemark formed during each of the first exposures, and thus apart from theprogramming at the first station, the system is automatic. Additionally,it is also possible to employ an arrangement wherein the projected imageis of a size smaller than the length of web at a particular exposurestation. This may be accomplished by employing a transparency or thelike having opaque shielding portions, or light shields masking out theunexposed portions of the web at each exposure station.

I -By a suitable control system, well known in the art, the programmingof the rate of exposures at the first station may be controlled in orderto maintain the distance between repetitive images at a minimum.Formation of an index or registration mark at the first station operatesautomatically to trigger the flash tube at each station as the photocellsenses the mark. As a result of this mode of operation, it is possibleto form images having a width corresponding to the web width and of anylength up to the length of web at each exposure station whilemaintaining the amount of unexposed paper between and around the imagesto a minimum.

It is of course understood that the exposing potrion of each station isconstructed so as to prevent light in the remaining stations from beingintroduced into any particular station, and thus spurious imagesresulting from extraneous light are avoided. An especially noteworthyadvantage of the system relates to the continuous tone printing in oneor more colors. An electrostatic latent image may be formed from acontinuous tone transparency or a continuous tone reflect copy byprocedures individually well known in the art. The image areas on thesurface of the image bearing members have a charge density which variesin accordance with the density of the image of the continuous tonetransparency or reflect copy. The quantity or density of the markingelement deposited on the electrostatic latent image varies in accordancewith the charge density to form a continuous tone image wherein thecolor density corresponds to the color density of the original.Additionally, by overprinting a continuous tone it is possible toprovide a fullcolor or multi-colored continuous tone print.

tionally, it is also possible to form only a two color print even thoughthe image bearing surface may be moved through five stations and thismay be conveniently accomplished by, for example, disabling the chargingunits at those stations in which no image formation is to occur.Additionally, the roller at each station corresponding to roller 78 maybe urged out of its normal position with respect to roller 79 such thatthe web is not treated with developer. Alternately, the developer may bedrained from the stations which are not to be used. Due to the nature ofthe developer and image bearing surface, as above described, backgrounddeposits will be maintained at a significantly low level if not entirelyeliminated.

As was previously noted, a registration unit is positioned in thestations other than the first station for providing predeterminedorientation of the subsequently formed images with respect to the firstformed image. Registration may be conveniently accomplished by aligningthe exposure units properly with respect to the Web travel and readingthe position of the index mark by the photocell detectors positioned inthe path of web travel along that portion of the web at which the indexor registration mark is formed. To accomplish reading of the index mark,a low intensity bulb preferably of red light is positioned in eachstation so as to provide a light source capable of rendering the indexmark visible to the photocell, and which does not interfere materiallywith the illumination generated by the flash source.

Since a red light may be employed for detection of the index mark, it ispreferred as a practical matter to deposit the red color if one isrequired, at any station other than the first station. The reason forthis will become apparent if one considers that if the index mark werered on an image bearing surface having a white background, the indexmark when exposed to red light would be indistinguishable from thebackground and therefore incapable of detection by the photocell.Accordingly, if a red color is to be deposited, it is preferred to havethe red color deposited in any order except the first, in order tofacilitate reading the index mark which may be illuminated by red light.In the alternative, if the index mark is red, a low level violet orultraviolet light may be em-.

ployed which does not materially interfere with the formation of theelectrostatic latent image and which is capable of illuminating theregistration mark to enable recognition by the photocell unit in anyparticular station.

As was previously discussed, the liquid developers of the presentinvention include components which are compatible insofar as the filmforming binders are concerned, and therefore, the ancillary andsecondary effects pre viously noted are substantially eliminated inorder to maintain background deposit at a minimum.

The development techniques as well as the operation of the squeegeedrying system may be more clearly understood with reference to FIG. 3which shows, in schematic form, the basic elements included in thedeveloping station as shown for example in FIG. 2, and which may beemployed with the single color apparatus shown in FIG. 1. A suitableimage bearing member of the type previously described having anelectrostatic latent image formed on the image bearing surface 151thereof is advanced to the developing station by suitable meansdiscussed above in connection with FIGS. 1 and 2. Liquid developer 153contained in a tank or other suitable receptacle 154 has at leastpartially immersed therein a developer roller 155 spaced from an idlingroller 156 to define a generally horizontal nip 157 therebetween havinga predetermined nip size.

The developer roller is arranged with respect to the receptacle 154 andthe developer 153 contained therein so as to dip into the same andcarry, the developer into contact with the surface 151 bearing theelectrostatic latent image as it is advanced between the nip 157. Theroller 155 may be of insulating or conductive material and may functionin one of two ways depending on the predetermined size of the nip 157.If the size of the nip 157 is less than the thickness of the imagebearing memher 159, the developer roller operates on a pressureprinciple which progressively brings the marking complexes into closecontact with the image bearing surface, and as a portion of the imagebearing surface is positioned at the point of minimum separation betweenrollers 156 and 155, a slight pressure is applied to urge the markingelements deposited in image configuration forcefully onto the surface ofthe image bearing member.

During downward movement of the roller, after contact with the imagebearing member, the marking elements not deposited on the image bearingsurface are redistributed through the developer due to the rotationalvelocity of the roller 155 with respect to the body of developer. Thedeveloped image is present in the area indicated generally at 160, andis progressively advanced to the sequeegee roller 165 positionedrelative to the roller 156 so as to define a generally vertical nip 167therebetween. Positioned between the roller 156, the image bearingsurface and the squeegee roller 165 is doctor blade 169 which projectsinto the nip 167 and whose surfaces 170 and 171 are spaced a relativelysmall distance from the surface of the image bearing member 150 and theouter surface of roller 165.

, The size of the nip 167 is preferably such that it is less than theaverage thickness of the image bearing member 150 so that the roller 165is maintained in intimate con tact with the image bearing surface andapplies pressure to the moving image bearing member to squeegee awaysubstantial amounts of the suspension medium which remain adhered to thesurface of the member 150. In order to prevent accumulation of thesuspension medium in the nip 167, the doctor blade 169 is positioned asabove described and arranged so that the lower portion thereof is incontact with the body of developer. In this manner, any significantamounts ofv suspension which tend to accumulate at the nip will beremoved, as formed, and conducted along the surfaces of the doctor blade169 to the body of developer 153.

An alternate mode of operation of the developing station shown in FIG.3, involves the use of a developer roller 155 positioned with respect toroller 156 so as to define a generally horizontal nip 157 having a nipwidth slightly greater than the average thickness of the image bearingmember 150. In this arrangement, the roller operates as an intermediateto apply liquid developer to the surface bearingan electrostatic latentimage and does not apply pressure as was the case with the arrangementpreviously described wherein the nip width is slightly less than theaverage thickness of the image bearing member. Regardless of the mode ofapplying the developer to the image bearing surface, the operation ofthe'squeegee roller is the same as that previously described.

As was described above, the squeegee roller is preferably constructed ofa material which exhibits electrical and polar properties similar tothat of the suspension medium. A roller constructed of vulcanizedlinseed oil or various synthetic rubbers have been found to operatesatisfactorily, and as a general rule, any resilient material which isunaffected by the suspension medium or components of the developer maybe utilized.

1 The enhanced advantages of a multi-color system, and of a squeegeesystem obtainable in accordance herewith are best realized by properlycoordinating the nature of the film forming binder with the componentsof the developer. In a multi-color system of the type above described,wherein an image bearing surface is successively charged, exposed anddeveloped by liquid developers of different colors, the stability of thefilm forming binder, and therefore the stability of the image bearingsurface is directly related to the interaction between the components ofthe developer and the surface of the image bearing member. Providing asystem of electrostatic photography wherein the developed image remainssubstantially fixed on the image bearing surface is in itself a highlydesirable goal. Additionally, the use of a developer which becomes 24substantially fixed to the image-bearing surface and is thereafterunaffected by repeated contact with a second or subsequent liquiddeveloper is highly advantageous in multi-color systems wherein a firstdepositing image may be exposed to as many as ten subsequent developingoperations.

As can be understood, care must be taken to avoid background depositswhich may interfere with subsequent exposure and developing operations,and in this connection, it is pointed out that the reduction orsubstantial elimination of the secondary effects such as viscousentrapment, and the like, previously noted operate advantageously tomaintain such background deposits at a uniform low level. Moreover, thecontrol of the electrical and chemical phenomena taking place at theinterface of the image bearing surface and the developer, and especiallythe ability to create a condition in which marking elements aremaintained, at least initially, in a semi-permanent transient state onthe surface of the paper advantageously allows the use of a squeegeeroller system which not only removes a substantial amount of thesuspension medium to allow polymerization of the resin component of themarking element, but also allows the use of a squeegee system whichcompresses the resin envelope to a substantially uniform thickness onthe surface of the image bearing member thereby allowing formation ofsubsequent images wherein the penetration of light onto theelectrosensitive coating or other suitable base member is uniform innature.

In accordance with the present invention, contamination or carry over ofdevelopers has been substantially eliminated by controlling electricaland chemical conditions at the developer-image bearing surface interfaceso as to eliminate, to a great degree, viscous entrapment of markingelements which may become redistributed in a second developer. Even ifsome marking elements were to be deposited, the fact that the elementsinclude resin components capable of being polymerized, and thusinsoluble in the suspension medium, provides another convenient way ofpreventing carry over of one developer into subsequent developerstations.

The system above described offers the advantage that the components ofthe developer composition and the resin surface of the image bearingmember are coordinated and correlated to Provide maximum stability ofthe resin surface during periods of repeated contact with the severaldeveloper compositions employed in multi-color work.

If desired, a counter-electrode or developer electrode may be employedwhich coacts with the field present as a result of the electrostaticlatent image to effect substantial elimination of halo or edge effectand to increase the rate of marking element deposition. An example of acounter-electrode which may be employed is one of the type disclosed incopending application Ser. No. 96,436, filed Mar. 17, 1961 aboveidentified, application Ser. No. 762,756, filed Sept. 23, 1958 andapplication Ser. No. 762,699, filed Sept. 23, 1958.

Moreover, various methods may be employed for forming an electrostaticlatent image, for example, devices wherein an electrostatic latent imageis formed directly on an insulator without necessarily involving thesteps of uniformly charging and exposing. Such systems are individuallywell known in the art.

The system above described constitutes a much improved system ofelectrostatic photography due to the fact that the developer isself-fixing, and equally as important is the fact that the image isformed and developed on an image bearing member which remainsdimensionally stable in the liquid developer. As a result of thestability of the electrosensitive member including a film forming binderwith the components of the liquid developer, it is possible to formmulti-color images by the procedure above identified.

While the methods and forms of apparatus herein described constitutepreferred embodiments of the invention,

25 it is to be understood that the invention is not limited to theseprecise methods and forms of apparatus, and that changes may be madetherein without departing from the scope of the invention which isdefined in the appended claims.

What is claimed is:

1. A process for sequentially forming a plurality of images on an imagebearing member including an image bearing surface comprising the stepsof sequentially forming a series of electrostatic latent-images on saidsurface, sequentially developing said images after formation thereofwith a liquid developer, each liquid developer including markingelements dispersed in an electrically insulating suspension medium, saidmarking elements of each developer including a liquid resin material forsubstantially adhering to said surface upon removal of said suspensionmedium, the suspension medium of each said developer being substantiallyfree of resin material other than that associated with the said markingelements so that resin material from the said developer is depositedessentially only where said marking elements are deposited, removing thesuspension medium adhering to said image bearing member after eachdevelopment operation to cause substantially permanent adhesion of themarking elements deposited during the said development operation, anddrying said image bearing member after the last development operation.

2. A process as set forth in claim 1 wherein each said developer is of adifferent color.

3. A process as set forth in claim 1 wherein each said latent image isof a different configuration.

4. A process as set forth in claim 1 wherein at least a portion of atleast one of said images overlaps at least a portion of some otherimage, said one and said other images being developed by two differentcolors, and said overlapping portion of said images being different incolor from said two different colors.

5. A process as set forth in claim 1 wherein at least one of said imagesis a continuous tone image, and wherein the marking elements of thedeveloper used for development thereof is deposited under the control ofsaid continuous tone electrostatic latent image.

6. A process as set forth in claim 1 wherein said image bearing memberincludes a surface composed in part of a polar polymeric material andwherein the suspension medium of said developer is non-polar in nature.

7. A process as set forth in claim 1 wherein said image bearing memberis in the form of a web, and wherein the portion thereof being developedis in facing contact with a developer while the opposite surface thereofis in contact with a roller.

8. A process as set forth in claim 1 wherein said suspension medium isremoved by pressure-contacting the image bearing member with a rotatingroller travelling at the same linear speed and in the same direction assaid image bearing member.

9. A process as set forth in claim 1 wherein said image bearing memberis a web maintained under tension, and wherein the surface of said webopposite the image bearing surface is maintained dry during developmentby contacting the opposite side with a roller.

10. A process as set forth in claim 1 wherein the image bearing memberis a web and wherein said process includes the further steps of formingan electrostatic latent image of a registration mark simultaneously withthe formation of the latent image of the original during the firstexposure operation, developing both said latent images, detecting saidregistration mark in at least one of said subsequent image formationoperations, and forming at least asecond image in registry with thefirst as controlled by said registration mark.

References Cited by the Examiner UNITED STATES PATENTS 2,676,100 4/1954Huebner 96-1 2,832,311 4/1958 Byrne 961 X 2,892,446 6/1959 Olden 96-12,907,674 9/1959 Metcalfe et al. 117934 X 2,961,921 11/1960 Lingg et al.8824 3,001,881 9/1961 Metcalfe et al 96-1 X 3,055,266 9/1962 Frantz etal. 8824 3,105,425 10/ 1963 Cerasani et al. 1.7 3,227,549 1/1966 Ullrichet al. 96-l J. TRAVIS BROWN, Primary Examiner.

C. L. BOWERS, Assistant Examiner.

1. A PROCESS FOR SEQUENTIALLY FORMING A PLURALITY OF IMAGES ON AN IMAGEBEARING MEMBER INCLUDING AN IMAGE BEARING SURFACE COMPRISING THE STEPSOF SEQUENTIALLY FORMING A SERIES OF ELECTROSTATIC LATENT IMAGES ON SAIDSURFACE, SEQUENTIALLY DEVELOPING SAID IMAGES AFTER FORMATION THEREOFWITH A LIQUID DEVELOPER, EACH LIQUID DEVELOPER INCLUDING MARKINGELEMENTS DISPERSED IN AN ELECTRICALLY INSULATING SUSPENSION MEDIUM, SAIDMARKING ELEMENTS OF EACH DEVELOPER INCLUDING A LIQUID RESIN MATERIAL FORSUBSTANTIALLY ADHERING TO SAID SURFACE UPON REMOVAL OF SAID SUSPENSIONMEDIUM, THE SUSPENSION MEDIUM OF EACH SAID DEVELOPER BEING SUBSTANTIALLYFREE OF RESIN MATERIAL OTHER THAN THAT ASSOCIATED WITH THE SAID MARTKINGELEMENTS SO THAT RESIN MATERIAL FROM THE SAID DEVELOPER IS DEPOSITEDESSENTIALLY ONLY WHERE SAID MARKING ELEMENTS ARE DEPOSITED, REMOVING THESUSPENSION MEDIUM ADHERING TO SAID IMAGE BEARING MEMBER AFTER EACHDEVELOPMENT OPERATION TO CAUSE SUBSTANTIALLY PERMANENT ADHESION OF THEMARKING ELEMENTS DEPOSITED DURING THE SAID DEVELOPMENT OPERATION, ANDDRYING SAID IMAGE BEARING MEMBERS AFTER THE LAST DEVELOPMENT OPERATION.